Hydrosilylation chemistry, typically involving a reaction between a silyl hydride and an unsaturated organic group, is the basis for synthesis routes to produce commercial silicone-based products like silicone surfactants, silicone fluids and silanes as well as many addition cured products like sealants, adhesives, and silicone-based coating products. Conventionally, hydrosilylation reactions have been typically catalyzed by precious metal catalysts, such as platinum or rhodium metal complexes.
Various precious metal complex catalysts are known in the art. For example, U.S. Pat. No. 3,775,452 discloses a platinum complex containing unsaturated siloxanes as ligands. This type of catalyst is known as Karstedt's-catalyst. Other exemplary platinum-based hydrosilylation catalysts that have been described in the literature include Ashby's catalyst as disclosed in U.S. Pat. No. 3,159,601, Lamoreaux's catalyst as disclosed in U.S. Pat. No. 3,220,972, and Speier's catalyst as disclosed in Speier, J. L, Webster J. A. and Barnes G. H., J. Am. Chem. Soc. 79, 974 (1957).
Although these precious metal complex catalysts are widely accepted as catalysts for hydrosilylation reactions, they have several distinct disadvantages. One disadvantage is that the precious metal complex catalysts are inefficient in catalyzing certain reactions. For example, in the case of hydrosilylations of allyl polyethers with silicone hydrides using precious metal complex catalysts, use of an excess amount of allyl polyether, relative to the amount of silicone hydride, is needed to compensate for the lack of efficiency of the catalyst in order to ensure complete conversion of the silicone hydride to a useful product. When the hydrosilylation reaction is completed, this excess allyl polyether must either be: (A) removed by an additional step, which is not cost-effective, or (B) left in the product which results in reduced performance of this product in end-use applications. Additionally, the use of an excess amount of allyl polyether typically results in a significant amount of undesired side products such as olefin isomers, which in turn can lead to the formation of undesirably odoriferous by-products.
Another disadvantage of the precious metal complex catalysts is that sometimes they are not effective in catalyzing hydrosilylation reactions involving certain type of reactants. It is known that precious metal complex catalysts are susceptible to catalyst poisons such as phosphorous and amine compounds. Accordingly, for a hydrosilylation involving unsaturated amine compounds, the precious metal catalysts known in the art are normally less effective in promoting a direct reaction between these unsaturated amine compounds with silyl hydride substrates, and will often lead to the formation of mixtures of undesired isomers.
Further, due to the high price of precious metals, the precious metal-containing catalysts can constitute a significant proportion of the cost of silicone formulations. Recently, global demand for precious metals, including platinum, has increased, driving prices for platinum to record highs, creating a need for effective, low cost replacement catalysts.
As an alternative to precious metals, certain iron complexes have been disclosed as suitable for use as hydrosilylation catalysts. Illustratively, technical journal articles have disclosed that Fe(CO)5 catalyzes hydrosilylation reactions at high temperatures. (Nesmeyanov, A. N. et al., Tetrahedron 1962, 17, 61), (Corey, J. Y et al., J. Chem. Rev. 1999, 99, 175), (C. Randolph, M. S. Wrighton, J. Am. Chem. Soc. 108 (1986) 3366). However, unwanted by-products such as the unsaturated silyl olefins, which are resulted from dehydrogenative silylation, were formed as well.
A five-coordinate Fe (II) complex containing a pyridine di-imine (PDI) ligand with isopropyl substitution at the ortho positions of the aniline rings has been used to hydrosilate an unsaturated hydrocarbon (1-hexene) with primary and secondary silanes such as PhSiH3 or Ph2SiH2 (Bart et al., J. Am. Chem. Soc., 2004, 126, 13794) (Archer, A. M. et al. Organometallics 2006, 25, 4269). However, one of the limitations of these catalysts is that they are only effective with the aforementioned primary and secondary phenyl-substituted silanes, and not with, for example, tertiary or alkyl-substituted silanes such as Et3SiH, or with alkoxy substituted silanes such as (EtO)3SiH.
Other Fe-PDI complexes have also been disclosed. U.S. Pat. No. 5,955,555 discloses the synthesis of certain iron or cobalt PDI dianion complexes. The preferred anions are chloride, bromide and tetrafluoroborate. U.S. Pat. No. 7,442,819 discloses iron and cobalt complexes of certain tricyclic ligands containing a “pyridine” ring substituted with two imino groups. U.S. Pat. Nos. 6,461,994, 6,657,026 and 7,148,304 disclose several catalyst systems containing certain transitional metal-PDI complexes. U.S. Pat. No. 7,053,020 discloses a catalyst system containing, inter alia, one or more bisarylimino pyridine iron or cobalt catalyst. However, the catalysts and catalyst systems disclosed in these references are described for use in the context of olefin polymerizations and/or oligomerisations, not in the context of hydrosilylation reactions.
Recently new and inexpensive Fe, Ni, Co and Mn complexes containing a terdentate nitrogen ligand have been found to selectively catalyze hydrosilylation reactions, as described in U.S. Patent Application Publication Nos. 20110009573 and 20110009565, the contents of which are incorporated herein by reference in their entirety. In addition to their low cost and high selectivity, the advantage of these catalysts is that they can catalyze hydrosilylation reactions at room temperature while precious metal based catalysts typically work only at elevated temperatures.
A restriction of these new non-precious metal-based catalysts, however, is that they are normally air and moisture sensitive, thus may not perform well if exposed to air or moisture prior to their use. For this reason, these catalysts are typically prepared and stored under inert conditions. Since it is difficult to keep reaction mixtures under inert conditions, such as a nitrogen blanket, on a commercial scale, the manufacturing of these catalysts in an industrial setting may be economically prohibitive. Accordingly, there is a need in the industry for non-precious metal-based catalysts that do not require manufacturing and storing under inert conditions.
Methods are known in the art to activate catalyst precursors in-situ. The most well known example is the activation of Ziegler-Natta catalyst by Methylaluminoxane (MAO) for the production of polypropylene from propene (Y. V. Kissin Alkene Polymerization Reactions with Transition Metal Catalysts, Elsevier, 2008, Chapter 4).
U.S. Pat. No. 5,955,555 discloses the activation of certain iron or cobalt PDI dianion complexes by polymethylaluminoxane (PMAO) for olefin polymerization. U.S. Pat. No. 4,729,821 discloses the in-situ activation of Ni-catalysts by applied electrical potentials for the hydrogenolysis of ethane and ethylene.
Martinez et al. demonstrated the in-situ activation of a [RuCl2(p-cym)]2 complex by phosphine ligands in a C—C bond formation reaction via C—H bond activation of aryl-compounds (J. Am. Chem. Soc, 2009, 131, 7887).
Yi et al. described the in-situ formation of cationic Ruthenium hydride complexes which catalyze the regioselective intermolecular coupling reaction of Arylketones and Alkenes involving C—H bond activation (Organometallics, 2009, 28, 426).
However, the in-situ activation of non-precious metal-based catalysts for hydrosilation reactions has not been known to the knowledge of the present inventors. Accordingly, there is a continuing need in the hydrosilylation industry for such methods. The present invention provides an answer to that need.